This invention relates generally to thermal barrier coatings for superalloy substrates and to a method of applying such coatings.
As gas turbine engine technology advances and engines are required to be more efficient, gas temperatures within the engines continue to rise. However, the ability to operate at these increasing temperatures is limited by the ability of the superalloy turbine blades and vanes to maintain their mechanical strength when exposed to the heat, oxidation, and corrosive effects of the impinging gas. One approach to this problem has been to apply a protective thermal barrier coating which insulates the blades and vanes and inhibits oxidation and hot gas corrosion.
Typically, thermal barrier coatings are applied to a superalloy substrate and include a bond coat overlayed by a ceramic top layer. The bond coat anchors both the top layer and itself to the substrate. The ceramic top layer is commonly zirconia stabilized with yttria and is applied either by the process of plasma spraying or by the process of electron beam physical vapor deposition (EB-PVD). Use of the EB-PVD process results in the outer ceramic layer having a columnar grained microstructure. Gaps between the individual columns allow the columnar grains to expand and contract without developing stresses that could cause spalling. Strangman, U.S. Pat. Nos. 4,321,311, 4,401,697, and 4,405,659 disclose thermal barrier coatings for superalloy substrates that contain a MCrAlY bond coat where M is selected from a group of cobalt, nickel, and iron. The MCrAlY bond coat is deposited by EB-PVD or vacuum plasma spaying. A more cost effective thermal barrier coating system is disclosed in Strangman, U.S. Pat. No. 5,514,482, which uses a diffusion aluminide bond coat. This bond coat is applied by electroplating platinum and diffusion aluminizing by pack cementation.
In commercially available thermal barrier coatings, the bond coat, whether MCrAlY or diffusion aluminide, is typically 1 to 5 mils thick and has a very low strength in comparison to the strength of the superalloy substrate. As a result, for design purposes the bond coats are considered to be non-load bearing.
At the high rotational speeds and temperatures typically encountered in today""s gas turbine engines, these bond coats have a difficult time in supporting the weight of the thermal barrier coating. In at least one instance, the Applicants have observed evidence that bond coating creep deformation permitted the zirconia thermal barrier coating to creep off the tips of turbine blades during high speed and high temperature operation.
One proposed solution to this problem, is to deposit the ceramic layer directly onto the oxide scale on the substrate. The disadvantage to this approach is that it requires additional air cooling to reduce the superalloy substrate metal temperature in order to achieve a satisfactory oxidation life.
Accordingly, there is a need for a thin, high strength bond coat that minimizes coating weight without incurring a creep strength penalty while inhibiting substrate oxidation.
An object of the present invention is to provide a superalloy article having a thin, high strength bond coat.
Another object of the present invention is to provide a thermal barrier coating system having a thin, high strength bond coat.
Yet another object of the present invention is to provide a method for applying such a bond coat.
The present invention achieves these objects by providing a thermal barrier coating for nickel based superalloy articles such as turbine engine vanes and blades that are exposed to high temperature gas. The coating includes a columnar grained ceramic layer applied to a platinum modified Ni3Al gamma prime phase bond coat having a high purity alumina scale. The preferred composition of the bond coat is 5 to 16% by weight of aluminum, 5 to 25% by weight of platinum with the balance, at least 50% by weight, nickel. The preferred thickness of the bond coating is 10 to 30 microns. A method for making the bond coat is also disclosed.